Due to the high cost of petroleum fuels and the necessity of producing heavier crude oils, large bore diesel engines used in ships and in stationary power engines on land have come to use heavier residual fuel and bunker fuel oil. Conventionally, these fuels were produced from bases that were not subjected to severe heat treatments such as residues obtained by subjecting petroleum oils to atmospheric or vacuum distillation. Today, with the introduction of apparatus for use in thermal cracking, processes such as visbreaking and delayed coking, an increasing amount of heavy bases thermally cracked at high temperatures are often used in combination with the fuel bases such as residues from atmospheric and vacuum distillation. Active efforts are also being made to run diesel engines with coal tar that is a by-product of carbonization of coal or tar resulting from naphtha cracking. These trends are reported on the paper No. D102 titling "The Impact of Changing Fuel Characteristics on Marine Diesel Engine Operation" at the International Congress on Combustion Engines (CIMAC) at Helsinki, 1981. However, all these attempts make fuel oil to result in increasing the C:H ratio, the residual carbon content, asphaltene content, and n-heptane insoluble content of the fuel.
Incidentally, conventional heavy fuel oils produced by mixing residual oils produced by atmospheric distillation of crude oils with residual oils generated in vacuum distillation, i.e., asphalt, have a residual carbon content of about 4 to 12 wt % and an asphaltene content of about 3 to 7 wt %, whereas heavy fuel oils produced by visbreaking process have a residual carbon content of 15 to 20 wt % and an asphaltene content of 8 to 15 wt %. Further, coal tar produced as a by-product by dry distillation of coal indeed reach a residual carbon content of 20 wt % or higher and an asphaltcnc content of 25 wt % or higher. Accordingly, fuel oils produced by mixing these oils become inevitably heavier and rich in the carbon content.
At present, medium speed trunk piston type diesel engines are widely used in main and auxiliary engines of large-scaled cargo boat and tanker and in stationary power engines on land. Conventionally, in lubricating cylinder and system of such engines, mineral oil type lubricating oils comprising as a base oil purified mineral oils having a viscosity corresponding to SAE 30-40, i.e., a dynamic viscosity at 100.degree. C. of 8 to 16 cSt, have been used. In other words, there have been commercially available lubricating oils in which 500 neutral oil and 150 bright stock oil, each of which can be obtained by purifying a lubricating fraction obtained from paraffinic or mixed base crude oils, are used as a base oil either alone or in admixture, and which are blended with various additives.
According to the experiment conducted by the present inventors, when a marine medium speed trunk piston type diesel engine was run with the above-described carbon-rich fuels in combination with diesel engine oils using conventional mineral oils as lubricating oil bases, the engine piston ring wore more rapidly and the filter on the purification system of the lubricating oil clogged more often than when the same engine was run with ordinary petroleum residual oil. It is known that engines wear due to the corrosive attack of sulfur in fuels. To prevent this, additives that increase the base number "alkalinity" of the lubricating oil are commonly used. However, this method has proved generally ineffective in preventing the wear of an engine that runs with low-sulfur fuels such as coal tar based fuels.
With respect to the influences by the alkalinity of lubricating oil and the type of cleaning dispersing agent for preventing the wear of large bore diesel engines in which high-sulfur heavy fuels are used, there is a report titling "Modern Marine Diesel Engine Lubricants and Their Development" at the 2nd International Lubricant Symposium (April 2-5, 1979, Cairo, Egypt). Further, U.S. Pat. No. 4,169,799 discloses that for preventing engines from the wear, a combined use of hydroxy chlorinated alkylphenyl sulfide and sulfurized over-based calcium alkylphenolate is effective. However, there have not yet been found any reports that the countermeasures for preventing the wear of engines and the clogging of cleaning filter of lubricant oil are examined from the viewpoint of the base oil construction of lubricating oils.